The present invention relates to the art of magnetic field gradient generation. It finds particular application in conjunction with establishing gradient magnetic fields in magnetic resonance imaging techniques and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in spectroscopy and other processes and apparatus in which accurately predictable magnetic field gradients are established or maintained.
In magnetic resonance imaging, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency pulses and magnetic field gradients are applied to the examination region. Gradient fields are conventionally applied as a series of gradient pulses with preselected profiles.
The gradient magnetic pulses are applied to select and encode the magnetic resonance signals. In some cases, the magnetic field gradients are applied to select one or more planes or slices to be imaged. Gradient field pulses are also applied for selectively modifying the uniform magnetic field to encode frequency and phase into the magnetization, hence the resonance signals in order to identify a spatial location.
The magnetic resonance signals are then processed to generate two or three dimensional image representations of a portion of the subject in the examination region. The accuracy of the resultant image representation, among other factors, is dependent upon the accuracy with which the actually applied magnetic field gradient pulses conform to selected gradient pulse profiles.
Conventionally, the uniform main magnetic field is generated in one of two ways. The first method employs a cylindric solenoidal shaped main magnet. The central bore of the main magnet defines the examination region in which a horizontally directed main magnetic field is generated. The second method employs a main magnet having an open geometry. Typically, the main magnet has opposing poles arranged facing one another to define the examination region therebetween. The poles are connected by a ferrous flux return path. This configuration generates a substantially uniform main magnetic field within the examination region. Open geometry main magnets have been able to solve important MRI issues such as, increasing patient aperture, avoiding patient claustrophobia, and improving access for interventional MRI applications. However, the design of gradient coils for generating the magnetic field gradients differs from that of the cylindrical type main magnets having horizontally directed main magnetic fields due to the orientation of the main magnetic field in relation to the gradient coils.
In the solenoidal coil type systems, conventional gradient coils included coils wound in a bunched or distributed fashion on a large diameter hollow right cylinder tube. Conventional bunch geometries include Maxwell or modified Maxwell pair for Z-gradient production and single or multi-arc golay saddle coils for X and Y-gradient production. The coils are normally wound in a series arrangement in a position to give a magnetic field profile with the desired linearity over a predefined volume. However, the large inductances which are typically in wound cylindrical gradient coils, limit the switching speed of the gradient magnetic field.
Planar gradient magnetic field assemblies have been developed for the cylindrical main magnets with horizontally directed fields. However, this type of planar gradient magnetic field assembly would not be directly compatible with main magnets having vertically directed fields. That is to say, in main magnets with horizontally directed fields, the direction of the main magnetic field is parallel to the planar surface of the gradient coil while in main magnets with vertically directed fields, the direction of the main field is orthogonal or transverse to the planar surface of the gradient coil.
In open magnet systems, the uniformity of the main magnetic field is dependent upon the separation gap between the magnetic poles and the dimensions of the magnetic poles. Typically, the ratio of the diameter of the magnetic poles to the distance between the poles is designed to allow for the greatest possible patient access. Generally, this results in the dimensions of the magnetic poles being similar to the dimensions of the bi-planar gradient coils typically employed with main magnets having an open geometry. In this case, the main magnetic field is fairly uniform within the imaging volume. However, non-uniformities and substantially non-zero transverse components in the field arise at larger axial and transverse locations near the periphery of the subject receiving region and the magnetic poles. Interactions between the non-uniformities of the vertical main magnetic field, and transverse components of main magnetic field, and the currents of the bi-planar gradient coils generates a net thrust force acting upon the coils. Depending upon the overall uniformity and strength of the main magnetic field, the magnitude of such thrust force can exceed 2,000 Newtons. Force of such magnitude can generate vibrations and mechanical stresses on the gradient structure, which can have an impact on the safety of the MRI scanner and can cause deleterious imaging effects such as ghosting and loss of resolution. Enlarging the dimensions of the magnetic poles to allow the gradient coils to reside in a more uniform area of the main magnetic field detracts from the purpose of having a main magnet with an open geometry in that it significantly reduces patient access.
The present invention contemplates a new and improved gradient coil configuration which overcomes the above-referenced problems and others.